Prepare release notes for BIND 9.16.0

WARNING: This is not your typical "prepare release notes MR".

Since BIND 9.16.0 is a stable version, three sections¹ of the release notes document have been rewritten accordingly. Also, the "Notes for BIND 9.16.0" section should only contain changes which do not apply to BIND 9.14.

I ordered entries within each section in (subjective) "decreasing importance" order. @each, @ondrej: you might want to voice your opinion in this regard.

Finally, this MR should not be merged into master, but rather into a new v9_16 branch, which in turn should be created right before tagging BIND 9.16.0. This is due to the fact that the modifications to various release note document sections should not be merged into master (instead, a version bump to 9.17 should be done).

@mnowak, @stephen: You might also want to take a peek at this MR so that we get the release notes right before tagging.

1. "Introduction", "Note on Version Numbering", "End of Life" ↩